This invention relates to an image reading apparatus that reads original images photoelectrically using a line CCD sensor. More particularly, the invention relates to an image reading apparatus that performs prescan of the images to calculate the image correcting conditions for fine scan of the images.
There has been known an image processing system in which the image recorded on a photographic film is read with an image reading apparatus having a reading sensor such as a CCD sensor. The resulting image data is subjected to correcting and various other image processing steps and recording of the image on a recording material, representation of the image on a display and other operations are subsequently performed. Film images (particularly negative images recorded on negative films) have great variations in density. Thus, in order to obtain image records or displays of the desired quality, the conventional image reading apparatus performs prescan (preliminary reading of the film image) to determine the reading conditions associated with the density and other factors of the film image (as exemplified by the quantity of light incident on the film image and the charge storage time of CCD) and then performs fine scan (reads the film image under the thus determined reading conditions).
When the color gradation of an original image is to be adjusted on the basis of the maximum density (Dmax) and minimum density (Dmin) of each color component that are determined from the prescanned image, the maximum density (Dmax) generally requires reading with a very high precision. However, image reading for prescan is difficult to achieve in a manner that is associated with the varying density ranges of the original image and, hence, the quantity of light and the storage time of a CCD sensor are often set as fixed conditions that are referenced to the base density of the original image (on a negative or reversal film). In this case, due to the limited dynamic range of the CCD sensor, the image that is read from the maximum density (Dmax) side of a high-density film is deteriorated in S/N ratio. Therefore, if a high-density film is to be read with an existing image reading apparatus that uses an area CCD sensor, precise reading of the maximum density (Dmax) must be ensured by performing a second prescan with the storage time varied from the first prescan.
However, due to the high cost of area CCD sensors, the conventional image reading apparatus depends on a single area CCD sensor for reading the three primary color components of one film image and this requires three prescans to be performed using a different color filter for each color component. In other words, in order to read a high-density film with the conventional image reading apparatus, three prescans must be performed with varying storage times and this has been painstaking and cumbersome operation.
Film with use of a lens is gaining wide acceptance because it is simple to use. However, images recorded by films with lens and other cameras that do not have a diaphragm have a tendency to become overexposed to produce xe2x80x9cover negativesxe2x80x9d. In order to read a film containing many over negatives, the number of prescan cycles is further increased to require more painstaking and cumbersome operations.
Under the circumstances, it has been proposed to use an inexpensive line CCD sensor comprising three line CCD elements that are individually equipped with filters for three primary color components. With such a line CCD sensor, the film images in a plurality of frames are transported for scanning to achieve simultaneous reading of the three primary color components in a continuous way, thereby reducing the cost of the image reading apparatus while realizing faster image reading. However, in order to perform repeated prescans for reading a high-density film in the manner described above, the line CCD sensor requires the film to be transported again for a second prescan and the problem of the painstaking and cumbersome operation becomes more serious than in the case of the area CCD sensor.
An object, therefore, of the present invention is to provide an image reading apparatus that uses an inexpensive line CCD sensor and which yet performs preliminary reading of an original image by a single prescan with the line CCD sensor that is set to an optimal storage time in accordance with the density of the original image. In this arrangement, optimal reading conditions for fine scan that are associated with the density and other factors of the original image (exemplified by the quantity of light incident on the film image and the charge storage time of the CCD sensor), and conditions for correcting the fine scanned image data and other processing conditions can be determined in an appropriate manner.
To attain the stated object, the present invention provides an image reading apparatus in which the image carried on an original undergoing relative movement is read by photoelectric with a line CCD sensor having a single array of CCD elements in a direction perpendicular to the relative movement of the original image, characterized in that prior to the fine scan of said original image by means of said line CCD sensor at high resolution, a single prescan at low resolution is performed on the scanning lines of the original image using at least two kinds of storage times of said line CCD sensor, in order to determine the reading conditions for the fine scan at the high resolution and the conditions for correcting the fine scanned image data.
In a preferred embodiment, the storage time of the line CCD sensor can be varied between odd-numbered scanning lines and even-numbered scanning lines.
The number of scanning lines for prescan using at least two kinds of storage times can be made different with each storage time.
If desired, the two kinds of storage times may comprise a standard storage time and a high-density image bearing original compatible storage time.
Further, at least one kind of image data, that was read by prescan using the two kinds of storage times, may be used for display on a monitor.
Further, and desired, at least one kind of image data that was read by prescan using the two kinds of storage times may be used for setup calculations.